Regarding gasoline engines, harmful components in the exhaust gases have been reduced securely by means of the strict regulations on the exhaust gases and the technological developments capable of coping with those. However, regarding diesel engines, the exhaust-gas conversion is difficult compared with the gasoline engines because of such unique circumstances that the harmful components are discharged as PMs (soot comprising carbon fine particles mainly, high-molecular-weight hydrocarbon fine particles, sulfur-system fine particles such as sulfates, and the like).
As exhaust-gas converting apparatuses for diesel engine that have been developed so far, the following have been known, and can be divided roughly as follows: trapping-type (or wall-flow) exhaust-gas converting apparatuses; and open-type (or straight-flow) exhaust-gas converting apparatuses. Among these, plugged-type honeycomb bodies that are made of ceramic (i.e., diesel PMs filters (hereinafter referred to as “DPFs”)) have been known as for one of the trapping-type exhaust-gas converting apparatuses. These DPFs are those which are completed by plugging both of the opposite ends of the cellular openings of ceramic honeycomb structures alternatively in a checkered manner, for instance; and they comprise inlet-side cells that are plugged on the exhaust-gas downstream side, outlet-side cells that neighbor the inlet-side cells and are plugged on the exhaust-gas upstream side, and cellular partition walls that demarcate the inlet-side cells and outlet-side cells; and they are one for suppressing the discharge of PMs by filtering exhaust gases and then collecting PMs therein with the cellular partition walls' pores.
In DPFs, however, since the exhaust pressure loss increases by means of the deposition of PMs, it is necessary to recover them by means of removing deposited PMs regularly by certain means. Hence, burning deposited PMs and then recovering DPFs forcibly have been carried out heretofore by means of such a method, and the like, in which a reducing agent, such as fuel, is added into exhaust gases; the exhaust gases are subjected to temperature rise by burning them with an oxidizing catalyst that is put in place on the upstream side of a DPF; and the resulting high-temperature exhaust gases are then supplied to the DPF.
However, at the time of running idly and during low-load/low-speed running, and the like, where low-exhaust-temperature state continues, no oxidation reaction occurs because the oxidizing catalyst or the like is not activated, and accordingly subjecting the exhaust-gas temperatures to temperature rise becomes difficult. Therefore, it becomes difficult to recover DPFs, and consequently the clogging of the DPFs has progressed.
Accordingly, it is possible to think of raising the exhaust-gas temperatures forcibly when the collection amount of PMs is judged to surpass a predetermined value. For example, data on the discharge amounts of PMs with respect to running conditions are stored in an ECU as a mapping data in advance, then a discharge amount of PMs is estimated from an accumulated value of running times, and then the resulting value is calculated cumulatively to estimate a collection amount of PMs. And, there is a method is available, method in which DPFs are recovered by increasing the exhaust-gas temperatures forcibly at a point of time when the resulting collection amount of PMs is judged to surpass a predetermined amount.
However, in the case of turning the data on the discharge amount of PMs with respect to running conditions into the mapping data, there is such a problem that the error is great. Moreover, as an index for the collection amount of PMs, it has been carried out as well to use the differential pressure between the pressure in front of a DPF and that in the rear thereof. In this method, however, since the value of limit differential pressure, which makes a standard for the judgment, changes greatly depending on the running conditions of engine, it is needed to store data on the limit differential pressures, which result from the respective running conditions, as another mapping data so that the data volume has become enormous. In addition, there is such a problem that the detection sensitivity is low in a range where the collection amount of PMs is less, because the relationship between the collection amount of PMs and the differential pressure is not in a linear relationship.
Hence, in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2005-325,771, a method is set forth, method in which an electric current or voltage that occurs in a secondary coil being wound around the outer periphery of a collector container is detected at the time of flowing an alternating electric current in a primary coil that is wound around the outer periphery of the collector container, and method in which a collection amount of PMs is then calculated from the resulting value. Since induced electromotive forces that correspond to collection amounts of PMs occur, it is possible to calculate the collection amount of PMs by detecting the electric current or voltage that occurs in the secondary coil.
By the way, DPFs are formed of ceramics, such as cordierite, in general, and are used in such a state that they are accommodated within a casing that is made of metal. In a case where the technique that is set forth in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2005-325,771 is applied to such DPFs, since electromagnetic waves are shielded by the casing, it is not possible to wind the primary coil and secondary coil around the casing's outer periphery. Therefore, the coils should be wound around a DPF inside the casing.
However, exhaust gases distribute within the casing, and accordingly it becomes such a high temperature as 1,000° C. at the time of recovering the DPF forcibly. Moreover, since being also an oxidizing atmosphere is multiplied adversely at the time of recovering it forcibly, the coils have deteriorated early. Therefore, it is not practical to apply the technique being set forth in the aforementioned gazette to the instance when estimating the collection amount of PMs on DPFs.
Moreover, in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 10-220,219, an exhaust-gas converting apparatus is proposed, exhaust-gas converting apparatus in which an amount of PMs is detected by measuring an intensity of electromagnetic wave with a microwave sensor. In this technique, an adhesion amount of PMs is detected as follows: a detection position of microwave is fixed by utilizing the facts that a dielectric constant and induction loss of a filter change when PMs deposit onto the filter, then the phase of microwave changes inside the filter, and eventually an intensity of the microwave changes; then an intensity of the microwave at that location is measured; and then the adhesion amount of PMs is detected by means of a change in the resulting intensity of the microwave.
However, since the filter's dielectric constant and induction loss come under the influence of temperatures, it is needed to do temperature correction; moreover, since an electromagnetic-field intensity is detected with the microwave sensor while utilizing a standing wave, not a direct phase-difference measurement, it is difficult to separate the effect of the microwave's attenuation by the one-point measurement, and accordingly there is such a problem that it cannot be detected with good accuracy. Consequently, it is needed to evaluate it in a comprehensive manner by carrying out the measurement at a plurality of locations; alternatively, it is needed to do troublesome evaluations using mapping data.
And, in the 2.45-GHz microwave that has been employed usually as a microwave, since the wavelength is 12 cm approximately so that the resolving power is low, it is not possible to detect the distribution of the collection amount of PMs, distribution whose size is this wavelength or less. Accordingly, it is not possible to prevent melt loss of the filter, melt loss which is caused as follows: PMs have deposited locally and thereby high temperatures have arisen locally, because of thermorunaway, at the time of the control for recycling the filter.    Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2005-325,771; and    Patent Literature No. 2: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 10-220,219